1. Field of the Invention
The invention relates to radar systems in general and more specifically to such systems including means for discriminating against unwanted receive signals at angles removed from the main beam.
2. Description of the Prior Art
In any directive antenna configuration, it is inevitable that sidelobes in addition to the main or intended beam are generated. In radar systems an effort is usually made to minimize the response of an antenna system along its sidelobes as compared to the response achieved in a main beam, usually by careful antenna design. When the antenna scans a volume of space and operates in the transmit/receive mode, some energy is inevitably transmitted at unintended angles via the sidelobes. Echoes from targets illuminated by this sidelobe energy can be received through the same sidelobe response. Considering that the inverse fourth power law applies, false angular indications are usually not a problem if the directive antenna has been carefully designed so that sidelobes are not an appreciable fraction of the main beam in amplitude.
In certain types of radar systems a much more difficult problem results from reception of unwanted signals at angles removed from the main radar beam due to sidelobe pick-up from independently radiating sources. That situation can be encountered when a hostile "stand-off" jammer illuminates the receiving antenna of a ground based radar system attempting to track a nearby target. The angle of the interfering source may be such that it coincides with a major sidelobe of the ground system antenna, in which case the interfering signal which is nearly always large compared to echo energy reflected by a target of interest presents the greatest difficulty to the ground radar system. Even if the interfering signal does not coincide with a significant ground antenna sidelobe, a high powered interfering source can frequently inject an undesirably large signal into the ground system.
In the prior art, effort has been expended toward the minimization of antenna sidelobes so that interfering signals at off-angles and other sources of false indications and responses are correspondingly minimized.
Still further, in the prior art, various electronic means have been implemented for minimizing the effect of sidelobes, especially where airborne beacon responses are involved. A so-called GTC (Gain Time Control) operates to reduce the gain of a receiver to replies close to the station, however, a far more satisfactory method of controlling interrogation beam width is the so-called SLS approach. Various SLS approaches are described in Chapter 38 of the text "Radar Handbook" by Merrill I. Skolnik (McGraw-Hill 1970). In general, the so-called SLS (Side Lobe Suppression) techniques rely on the use of a known coded pulse format as from an airborne beacon, for example. A hostile interfering source obviously does not accommodate the ground radar by providing a signal format permitting the ground implementation of most SLS techniques.
The so-called monopulse technique in which the antenna of the ground station provides interferometer operation (i.e., sum and difference patterns) can be employed to steepen the effective ground antenna beam skirts and thereby discriminate against received signal energy at off-angles.
The particular problem represented by the hostile interfering source has not been sufficiently effectively addressed in the prior art. The manner in which the present invention provides greatly improved rejection of off-angle receive signal energy will be understood as this description proceeds.